1. Field of Invention
The present invention relates to a constant ON-time or constant OFF-time switch power converter and a control circuit thereof; particularly, the present invention relates to such a constant ON-time or constant OFF-time switch power converter which can achieve fixed-frequency control without requiring a complicated phase lock loop (PLL), and a control circuit thereof.
2. Description of Related Art
FIG. 1 shows a conventional switching power converter. A power switch control circuit 11 controls at least one power switch (referring to FIGS. 2A-2H) within a power stage circuit 12, to convert an input voltage Vin to an output voltage Vout, and to provide an output current Iout when it is required by a load circuit (not shown). The power stage circuit 12 is for example but not limited to a synchronous or non-synchronous buck, boost, inverting, or buck-boost power conversion circuit as shown in FIGS. 2A-2H. Referring to FIG. 1, a feedback circuit 13 generates a feedback signal related to the output voltage Vout, and sends the feedback signal to the power switch control circuit 11, so that the power switch control circuit 11 controls the power stage circuit 12 according to the feedback signal to regulate the output voltage Vout or the output current Iout to a target.
Typically there are two control modes for the power switch control circuit 11 to control the power stage circuit 12, i.e., the fixed-frequency control mode and the variable-frequency control mode. As an example, assuming that the power stage circuit 12 is the non-synchronous buck power conversion circuit of FIG. 2B, FIG. 3A shows the waveform of the power switch gate signal in the fixed-frequency control mode, wherein in a heavy load condition (the load circuit consuming a relatively higher current), the ON-time of the power switch is ton1, whereas in a light load condition (the load circuit consuming a relatively lower current), the ON-time of the power switch is ton2. The cycle period is T, which is the same in both the heavy load condition and the light load condition, but the ON-time of the power switch changes in response to different load conditions. FIG. 3B shows the waveform of the power switch gate signal in a constant ON-time control mode of the variable-frequency control mode (also assuming that the power stage circuit 12 is the non-synchronous buck power conversion circuit of FIG. 2B), wherein the ON-time of the power switch is ton, which is the same in both the heavy load condition and the light load condition, but in the heavy load condition, the cycle period is T1 which is relatively shorter and the frequency is faster, whereas in the light load condition, the cycle period is T2 which is relatively longer and the frequency is slower, to respond to different load conditions. FIG. 3C shows the waveform of the power switch gate signal in a constant OFF-time control mode of the variable-frequency control mode (also assuming that the power stage circuit 12 is the non-synchronous buck power conversion circuit of FIG. 2B), wherein the OFF-time of the power switch is toff, which is the same in both the heavy load condition and the light load condition, but in the heavy load condition, the cycle period is T3 which is relatively longer and the frequency is slower (so the ON-time is relatively longer), whereas in the light load condition, the cycle period is T4 which is relatively shorter and the frequency is faster (so the ON-time is relatively shorter), to respond to different load conditions.
In the constant ON-time and OFF-time control modes of FIGS. 3B and 3C, although the ON-time or OFF-time of the power switch is a constant and the frequency is therefore variable, in many applications, it is preferred to properly set the ON-time or OFF-time so that the power switch operates by a fixed frequency. Still assuming that the power stage circuit 12 is the non-synchronous buck power conversion circuit of FIG. 2B, if the input voltage Vin and the output voltage Vout are both known, a proper duty ratio (which is equal to the ON-time divided by the cycle period) of the power switch can be determined according to the relationship between the output voltage Vout and the input voltage Vin; hence, under a given frequency, a proper ON-time or OFF-time can be calculated, and the power switch can operate by a fixed frequency (which is the given frequency) according to this calculated ON-time or OFF-time.
To achieve the aforementioned fixed-frequency control, the switching frequency of the power switch has to be first aligned to the given frequency, and then, as the power switch operates according to the calculated ON-time or OFF-time, the switching frequency of the power switch will naturally be locked to the given frequency. However, if the switching frequency of the power switch is not first aligned to the given frequency, although a proper ON-time or OFF-time is calculated, the power switch may not be able to operate by the desired fixed frequency. In the prior art, a typical solution to this is to provide a PLL to align the switching frequency of the power switch to the given frequency, such as in U.S. Pat. No. 6,476,589. However, a PLL is a complicated, high cost, and large size circuit.
In view of the above, the present invention proposes a constant ON-time or constant OFF-time switch power converter which can achieve fixed-frequency control without requiring a complicated phase lock loop (PLL), and a control circuit thereof.